Many oil and gas wells in the past have suffered from the physical phenomenon known as "water coning". This phenomenon can seriously affect the rate of hydrocarbon production in a well, even to the point of rendering a perfectly good well non-commercial to produce because of massive water cut or water to oil ratio in the produced well fluid. In water coning, there is a hydrocarbon bearing formation (oil or gas or both) without an impermeable barrier or layer of earth formation between the water and oil bearing layers. That is to say, there is an oil/water contact surface in the well. This situation is common even in the most prolific producing formations wherein an underlying "water drive" can be a main pressure source for hydrocarbon production. When hydrocarbon is withdrawn from the formation by perforations above the oil/water conduct the underlying water is drawn up into the void pore spaces by the viscous forces acting on the fluid. Water thus invades the hydrocarbon bearing portion of the formation, and can do this even up to and beyond level of the production perforations. This can severely impede the rate at which the hydrocarbon can thereafter be withdrawn from the production perforations. The local rise (near the perforations) in the oil/water contact surface has a conical shape, being highest near the borehole and tapering off away from the borehole, thus leading to the name "water coning".
In the prior art, it has been proposed to complete such a well so as to mitigate the effect of water coning by producing into the wellbore, a significant volume of water, drawn thereto by a second set of perforations in the well casing and located below the oil/water contact surface while concurrently producing hydrocarbon through perforations in the well casing above the oil/water contact surface. The water production below the oil/water contact surface has the effect of suppressing the creation of the water cone. This protects the hydrocarbon zone near the casing from being invaded by the underlying water. The downward viscous forces imposed on the formation fluid created by withdrawing water below the oil/water contact surface tend to balance the upward viscous forces created by withdrawing hydrocarbon from the formation above the oil/water contact surface. Hydrocarbons may therefore be withdrawn from the formation through the hydrocarbon production perforations at rates significantly greater than is possible under conditions where a water cone has invaded the hydrocarbon bearing formation. This technique has come to be known in the art as In-Situ Gravity Segregation or IGS.
The present invention, however, provides methods for well completion and production allowing the IGS technique to be practiced using a single string of production tubing and conventional artificial lifting technique. Also, using the techniques to the present invention it is possible to produce and induce different FBHP.sub.(oil) and FBHP.sub.(water) pressures and lift to the surface fluids from two perforated intervals having different bottom hole flowing pressure (BHFP's) while using only a single fluid conduit to the surface. This may be accomplished whether or not the upper set of perforations' BHFP is greater or less than the lower perforations' BHFP.